1. FIELD OF THE INVENTION
The invention relates to band surveillance receivers generally and more particularly to such receivers of the compressive type which convert discrete frequencies to corresponding pulse times for discrete identification.
2. DESCRIPTION OF THE PRIOR ART
In the prior art, compressive receivers are known. In general these have been implemented according to analog techniques. One such disclosure is contained in U.S. Pat. No. 2,882,395. The device described therein makes use of a linearly swept local oscillator feeding a mixer which also accepts signals in the band of surveillance. Resulting intermediate frequency signals are thereby time separated or "coded" according to the frequency of each received signal. When applied to a "dispersive network", i.e. a network which provides a time delay which is a linear function of frequency, the received frequencies are effectively spread along a time base and arranged in order of frequency. In a surveillance receiver, calibration of this time-base permits the identification of the particular frequency of every signal being received within the band of interest at any one time so that other equipment can be timed to monitor or otherwise process any such received signal.
One of the greatest difficulties in instrumenting a system such as described in the aforementioned U.S. Pat. No. 2,882,395, is the problem of obtaining adequate linearity of the frequency sweep "FM modulation". Small errors of departures from perfect linearity of the frequency sweep produce relatively large errors in signal relative timing at the output of the dispersive delay line. In fact, for signals at the input of the surveillance receiver which are relatively close in frequency, non-linearities in the frequency variation function can deteriorate the frequency resolving capability of the system to such an extent that close signals may not be separated at all when they appear as identification pulses at the output of the dispersive delay line.
A system disclosed in U.S. Pat. No. 4,038,612 provides substantial relief from that problem. In that patent, a zero crossing detector monitors the FM sawtooth of the system and detects errors from idealized crossover points throughout the FM sawtooth. Errors are integrated and fed back to the modulating sawtooth waveshape such that imperfections of that waveshape as well as inherent non-linearities in the voltage controlled oscillator which actually produces the FM ramp are compensated for.
While constituting a significant improvement, the system of U.S. Pat. No. 4,038,612 is nevertheless still an analog system insofar as the generation of the FM ramp is concerned. As long as the basic sawtooth FM ramp is employed in the compressive surveillance receiver configuration, the problem of its linearity continues to be a critical design consideration.
The linearizing of the FM ramp is a costly operation since the linearity requirement is severe. Moreover, even perfect linearity of the FM ramp leaves dispersive delay line linearity deviations essentially uncorrected.
The manner in which the present invention deals with the disadvantages of the prior art by providing a system based on a unique modulation and compression arrangement not requiring a linear FM ramp or a dispersive delay line will be described as this specification proceeds.